Recent advances in optical networking have led to the development of reconfigurable optical add/drop multiplexers (ROADMs) and remotely reconfigurable optical add/drop multiplexers (RROADMs), which provide carriers with the ability to remotely select one or more wavelengths sent from any node to any other node on a core ring. This simplifies and reduces the amount of planning and provisioning work required.
FIG. 1 illustrates a generic prior art RROADM based node 100 having two RROADMs 101 and 101a, wherein optical fibers 103 and 103a carry wavelength division multiplexed (WDM) optical signals from an adjacent node and optical fibers 105 and 105a carry the WDM signals around a network to the next adjacent node. Typically, nodes are arranged in a ring configuration with a RROADM (101 and 101a) configured for each direction.
Each generic RROADM implementation 101 and 101a includes an optical drop module 107 where one or more selected wavelengths of the WDM signal are received at input port 104 and diverted to drop port 111. The WDM optical signal containing the remaining wavelengths which have not been diverted, continue on through path 113 to add module 115 where one or more selected wavelengths from add port 117 are combined with the WDM optical signal on the through path and presented to output port 119. In a remotely reconfigurable OADM (RROADM), selection of wavelengths to add and drop can be controlled remotely.
In many implementations of RROADMs, RROADM through losses and/or fiber losses are sufficient to require an ingress optical amplifier 121 at the RROADM input and an egress optical amplifier 123 at the RROADM output.
In optical fibers, waveforms of signals broaden over long distances as a result of chromatic dispersion. This phenomenon increases as network speeds and span lengths increase and can result in transmission errors if the transponder dispersion tolerance is exceeded.
Many optical networks require span lengths beyond the dispersion tolerance of WDM transponders and therefore the chromatic dispersion of the fibers require compensation. This is typically achieved through the use of dispersion compensation modules (DCMs) having a fixed amount of compensation (for example 20, 40, 60 or 80 km). For long haul networks, both static and reconfigurable, per span compensation is required, most spans requiring multiple DCMs.
FIG. 2 illustrates a prior art RROADM implementation where dispersion compensation is required. The node 200 of FIG. 2 uses RROADMs 101 and 101a of FIG. 1, with the addition of DCMs 201 and 203. DCM 201 provides dispersion compensation on the incoming span 103 (and 103a). DCM 203 provides dispersion compensation on the outgoing span 105 (and 105a). In practice either DCM 201 or DCM 203 or both are used depending on the amount of dispersion compensation required, the compensation values of the DCMs available and other network planning requirements. In most RROADM architectures, the optical losses in the DCMs require additional optical amplification as shown by associated optical amplifiers 205 and 207.
In some installations, the DCM 203 can be installed before the egress amplifier 123 and can obviate the need for additional amplifier 207. It is also possible to install DCM 201 between the ingress amplifier 121 and input port 104 although this can have a detrimental effect on optical signal-to-noise ratios (OSNR).
Metro and regional networks have quite different dispersion issues compared to long haul networks. In metro/regional networks, many spans may be too short to require individual span compensation but longer signal paths traversing multiple spans often exceed transponder dispersion limits. This problem becomes more noticeable when exploiting the ability to remotely reconfigure RROADMS to route wavelengths from any source to any destination, without exceeding total dispersion limits for the signal paths. Transponder dispersion limits have both positive and negative bounds (for example, −20 to +80 km, of equivalent SMF-28 fiber dispersion). It is therefore important not to overcompensate a signal path. Maximum dispersion compensation on any span is limited, so that for example, on a span of 25 km, a maximum allowable compensation would be 45 km to remain within transponder minimum dispersion limit of −20 km. Generally, average compensation values over multiple spans must remain close to the average span lengths, so that signal paths traversing multiple spans have dispersion within transponder dispersion limits. Traditionally, metro/regional networks use tuneable DCMs or adaptive dispersion compensation to ensure dispersion remains within limits while still being able to accommodate the flexibility of any node to any node signal routing. On the other hand, if static DCMs are used, many small value DCMs are needed so as not to overcompensate signals traversing only a single span.
These traditional techniques have the disadvantage of being costly to implement. Tuneable DCMs and adaptive dispersion compensation are much more costly than static DCMs.
It is therefore desirable to avoid using expensive tuneable and adaptive DCMs and to use static DCMs if possible, and to minimize the number of DCMs used. As well, it is desirable to minimize the number of different value static DCMs required, in order to reduce inventory requirements.